Apparatus and method for paper position sensing using transparent transport belt

Abstract

A printing apparatus has a print head and a transport mechanism including a continuous transparent belt for transporting a sheet medium past a print head. The apparatus includes a through optical sensor operable to direct a sensing beam through the belt for sensing the position of the transported sheet medium when the sheet medium breaks the beam.

What is claimed is: 
     
       1. A printing apparatus having a print head, a transport mechanism comprising a continuous belt for transporting a sheet medium supported on and retained substantially in a fixed position relative to the belt during transport by the belt in a transport direction for printing an image thereon by the print head, the belt being at least partially transparent, and at least one through optical sensor positioned to direct a sensing beam through the belt for sensing the position of the transported sheet medium supported on the belt upon the sheet medium breaking the beam. 
     
     
       2. A printing apparatus as claimed in  claim 1 , the optical sensor comprising a source and a detector, the source located on one side of the belt, the detector located on the other side of the belt. 
     
     
       3. A printing apparatus as claimed in  claim 2 , the belt extending horizontally, the source and the detector vertically aligned. 
     
     
       4. A printing apparatus as claimed in  claim 1 , the source and the detector located on the same side of the belt with the source located laterally adjacent or downstream of the detector in the transport direction, a beam reflector located on the other side of the belt for directing a light beam sent by the source and passing through the belt to the reflector back through the belt to the detector when the light beam from the source is not broken by the presence of a sheet medium, the source positioned to direct said sent light beam generally perpendicularly through the belt, the detector configured to direct the reflected light beam generally perpendicularly through the belt. 
     
     
       5. A printing apparatus as claimed in  claim 1 , having a plurality of such print heads spaced from one another in a transport direction, the transport mechanism operable to transport a sheet medium tacked to the belt in the transport direction for printing partial images thereon successively by the respective print heads, at least some of the print heads having a respective through optical sensor associated therewith. 
     
     
       6. A printing apparatus as claimed in  claim 5 , at least one of the through beam sensors having one of a source and a detector of the through optical sensor mounted on the print head. 
     
     
       7. A printing apparatus as claimed in  claim 1 , the at least one optical though sensor being a sheet medium launch detector for detecting a leading edge of the sheet medium as it is launched onto the belt, the optical through beam sensor having a first signal output, the apparatus further comprising a measuring device for measuring the movement of the belt in the transport direction, the measuring device having a second signal output, and a signal processing device for processing the signals from the optical through beam sensor and the measuring device to derive the position of a sheet medium transported by the belt. 
     
     
       8. A printing apparatus as claimed in  claim 7 , the belt driven by a pulley system including a drive shaft, the measuring device including a shaft encoder for measuring rotations of the drive shaft. 
     
     
       9. A printing apparatus as claimed in  claim 1 , the optical through beam sensors each having an output signal, and a signal processing circuit to process the output signals to determine an instance of a sheet medium transported by the belt not being detected by one of the optical through beam sensors when, as a result of prior monitoring of the sheet medium position and the measuring of the movement of the belt, the sheet medium is expected to be detected by such optical through beam sensor. 
     
     
       10. A printing apparatus as claimed in  claim 1 , having a plurality of such optical through beam sensors extending transversely of the transport direction. 
     
     
       11. A printing apparatus as claimed in  claim 10 , the transversely extending optical through beam sensors having output signals, and a signal processing circuit to detect the signals from the transversely extending optical sensors and to identify skew in the position of a sheet medium in relation to the belt. 
     
     
       12. A method of sensing a sheet medium that is supported on, and retained substantially in a fixed position relative to, a continuous belt transporting the sheet medium in a transport direction past a print head for printing on the sheet medium, the belt being at least partially transparent, the method comprising directing a sensing beam of an optical through beam sensor through the belt and detecting the position of the sheet medium by detecting when a leading edge of the sheet medium supported on the belt breaks the sensing beam. 
     
     
       13. A method as claimed in  claim 12 , further comprising directing the sensing beam from an optical source located on one side of the belt through the belt to a detector located on the other side of the belt. 
     
     
       14. A method as claimed in  claim 12 , further comprising directing the sensing beam from an optical source located on one side of the belt though the belt to a reflector located on the other side of the belt, reflecting the sensing beam from the reflector back through the belt to a detector located on said one side of the belt upstream of the optical source, the sensing beam being directed generally perpendicularly through the belt, the reflected light beam being reflected generally perpendicularly through the belt. 
     
     
       15. A method as claimed in  claim 12 , further comprising transporting the sheet medium retained at the belt in the transport direction past a plurality of print heads spaced from one another in a transp ort direction for printing partial images thereon successively by the respective print heads, at least some of the print heads having a respective through optical sensor associated therewith, and operating the optical sensors to track the position of the sheet medium during its transport past the print heads. 
     
     
       16. A method as claimed in  claim 15 , further comprising operating at least one of the optical sensors to send a sensing beam from an optical source located on one side of the belt through the belt to a detector located on the other side of the belt. 
     
     
       17. A method as claimed in  claim 15 , further comprising operating one of the sensors to detect a leading edge of the sheet medium as it is launched onto the belt thereby to produce a first signal output from said optical through beam sensor related to a detected leading edge of the sheet medium, operating a measuring device for measuring the movement of the belt in the transport direction to produce a second signal output from the measuring device related to the position of the belt, and processing the first and second signal outputs to derive the position of the sheet medium transported by the belt. 
     
     
       18. A method as claimed in  claim 15 , further comprising operating the optical through beam sensors to produce respective output signals and processing the output signals to determine an instance of a sheet medium transported by the belt not being detected by one of the optical through beam sensors when, as a result of prior monitoring of the sheet medium position and the measuring of the movement of the belt, the sheet medium is expected to be detected by such optical through beam sensor. 
     
     
       19. A method as claimed in  claim 12 , further comprising operating a plurality of the optical through beam sensors extending transversely of the transport direction to produce a respective plurality of output signals, and processing the output signals to identify skew in the position of a sheet medium in relation to the belt.